1. Field of the Invention
This invention relates to a thermal response switch used for a surge absorbing circuit and to the circuit suitable for protecting electronic parts used for communication equipment, such as, telephones, facsimiles, telephone switchboards, modems and the like, from surge voltages and continuous overvoltages or overcurrents. More particularly, it relates to a surge absorbing circuit which prevents continuous overvoltages or overcurrents from flowing in electronic devices and absorbs surge voltages applied to the electronic devices.
2. Description of the Prior Art
In the prior art, a surge absorbing element, e.g., a gas charge tube, used in a surge absorbing circuit is parallel connected to the electronic device to be protected via a pair of input lines of the electronic device, and is designed to operate at a higher voltage than the operating voltage of the electronic device. Such a prior art surge absorbing element is a resistor having a high resistance when the voltage applied thereto is lower than the discharge voltage thereof and a resistance tens of ohms lower, when the voltage applied thereto is higher than the discharge starting voltage thereof. Accordingly, when surge voltages, such as, lighting surges, etc., are instantaneously applied to an electronic circuit including the surge absorbing element and the electronic device, the surge absorbing element discharges to suppress the surge voltages, and serves to protect the electronic device from the surge voltages.
However, when an overvoltage or overcurrent, e.g., due to an accident, is continuously applied to the electronic circuit, a certain amount of current continuously flows through the surge absorbing element. This results in the surge absorbing element being heated to high temperatures. The heat radiating from the surge absorbing element can cause the protected electronic device, as well as other electronic devices surrounding the surge absorbing element, to catch fire.
A typical example would be an accident wherein the input, i.e., signal or communication, lines of the electronic device contact the power lines thereof. While it does not usually happen that such accidental overvoltages are continuously applied to the surge absorbing element, to achieve maximum safety, it has recently become desirable to take additional safety measures to avoid such accidental problems and the potential fires caused thereby. As an example, UL (Underwriter's Laboratories Inc.) of the U.S.A prescribes a safety standard for surge absorbing elements so that they do not cause fire or electrical shock in communication equipment surrounding the surge absorbing element when continuous overvoltages or overcurrents are applied.
PCT Patent Application No. JP90/01006 discloses a surge absorber comprising a surge absorbing element used for suppressing the surge voltages and a metal wire connected in series to the surge absorbing element to prevent abnormal heating of the surge absorbing element. This surge absorber is shown in FIGS. 4 and 5.
In the surge absorber of PCT Application No. JP90/01006, a first lead 17, a second lead 18 and a third lead 19, are attached to a base plate 16. One end of metal wire 15 having spring elasticity is welded to an end of the first lead 17. A surge absorbing element 14 is connected between the second lead 18 and the third lead 19 through lead wires 14a and 14b. The metal wire 15 is attached, as by a weld, to one end of the first lead 17, and is bent in a spring-loaded position in the direction of the surge absorbing element 14. The other end of the bent spring-loaded wire 15 is attached by solder 28 to one end of lead wire 14a, which is connected to the second lead 18. The metal wire 15 and the surge absorbing element 14 are encased within casing 24, which is attached to base plate 16.
As shown in FIG. 5, the first lead 17 is connected to an input line 11a of an electronic device 10, the second lead 18 is connected to an input line 11b of electronic device 10, and the third lead 19 is connected to electronic device 10. While the metal wire 15 does not blow, i.e., disconnect, when a surge voltage is instantaneously applied to the above surge absorber, it does blow to prevent a current flowing to the surge absorbing element 14 when subjected to large current at an overvoltage.
In particular, when a small current at an overvoltage flows, solder 28 is melted by heat generated by both the current and the surge absorbing element 14, and wire 15 releases from its spring-loaded position and disconnects from its attachment to lead wire 14a thereby preventing the current from flowing to the surge absorbing element 14. However, if the surge voltages are repeatedly applied to the surge absorber, wire 15 loses its spring elasticity because it is repeatedly annealed by the heat of the surges. As a result, it may not spring back and detach from lead wire 14a.
Consequently, the small current at the overvoltage continues to flow into the surge absorbing element, the surge absorbing element is abnormally heated by the current and causes the electronic device as well as other electronic devices surrounding the surge absorber to catch fire. For the above reasons, this surge absorber cannot pass the U.L. safety standard.
Moreover, when the surge absorbing circuit has become opened by the blowing of a metal wire or melting of the solder, it is troublesome to remove and replace. Furthermore, it is difficult to visually detect the blown state because of the casing which covers the surge absorber.